Ensure git_futils_mkdir won't mkdir root

This makes sure that git_futils_mkdir always skips over the root
directory at a minimum, even on platforms where the root is not
simply '/'.  Also, this removes the GIT_WIN32 ifdef in favor of
making EACCES as a potentially recoverable error on all platforms.
Merge pull request #1634 from arrbee/alternative-mkdir-fix

Ensure git_futils_mkdir won't mkdir root